鄂尔多斯盆地东胜地区砂岩型铀矿成因探讨

Discussion on Genesis of Sandstone-Type Uranium Deposits in Dongsheng Area, Ordos Basin

鄂尔多斯盆地是一个铀、煤、油、天然气共存的沉积盆地。通过电子探针、流体包裹体测温、扫描电镜、偏反光显微镜、煤的微量元素等方法综合分析,认为盆地北部东胜地区的侏罗系砂岩型铀矿属于与天然气还原有关的低温热液型铀矿床,其证据有:1铀石主要分布于充填在孔隙中或微裂缝内的黄铁矿晶体中,其次充填在伊/蒙混层中和钾长石溶孔内的黄铁矿中。2铀石中UO2约占70%,SiO2约为18%,另有极少量的Fe、Ca、Se、Pb、Cr、Ti等伴生元素。3包裹体流体势显示盆地中部的天然气向北东方向运移并大量散失,出现大量被漂白的砂岩;盆地内主砂体展布方向与流体运移方向一致。4天然气包体中见石盐子晶,砂岩胶结物和矿物表面见较多细小盐粒,次生包裹体温度多在140～170℃。5富含黄铁矿、黄铜矿、方铅矿、闪锌矿等低温热液矿物;具有水热蚀变现象(碳酸盐化、硅化、水云母化、黄铁矿化等)。6燕山运动期间岩层进入表生成岩阶段,利于富氧的地表水和来自地层深部的低温热液进入砂岩中,可见褐铁矿溶蚀、重晶石充填于钾长石溶蚀孔现象。

Ordos basin is a typical deposition basin in which the multi-energy resources-oil, gas, coal and uranium coexist. The comprehensive analysis by means of electron probing, fluid inclusions data , scanning electron microscopy, microscope and trace element data from coal demonstrates that the Jurassic sand-type uranium deposit in Dongsheng area, northern Ordos basin belongs to low temperature hydrothermal uranium deposit reduced by natural gas. The evidence is that ： ① uranous sandstone is mainly distributed in the pyrite filling the pores and micro cracks; it can also be found filling I/S and K-feldspar solution pores in the pyrite; ② the uranous sandstone is composed of UO2 （about 70%）, SiO2 （about 18% ）, and a few associated elements such as Fe, Ca, Se, Pb, Cr and Ti ; ③ Fluid potential of the fluid inclusion shows that the natural gas in the mid-basin migrated to the northeast and lost, while the bleached sandstone appeared in the migration passage; distribution of the main sandbodies in the basin was consistent with the direction of fluid migration.④ Vapor hydrocarbon inclusions contain rock salt and a lot of small salt grain could be seen on cements and minerals in sandstone ,the homogeneous temperatures of 140 ～170 ℃Pyrite , chalcopyrite, galenite, blende etc. mineral association ; thermal fluid alteration phenomenon （carbonation, silicion, illation and pyrition etc. ）.⑥Yanshan movement made the Zhiluo formation into the epidiagenetic stage, this is propitious to the upper oxygenous water and low temperature hydrothermal fluid from deep formation flowing the sandstone ,it is obvious that limonite dissolved and barite filling the K-feldspar solution pores.